By Cobalt Carbonyl Catalysts

An early attempt to hydroformylate butenediol using a cobalt carbonyl catalyst gave tetrahydro-2-furanmethanol (95), presumably by aHybc rearrangement to 3-butene-l,2-diol before hydroformylation. Later, hydroformylation of butenediol diacetate with a rhodium complex as catalyst gave the acetate of 3-formyl-3-buten-l-ol (96). Hydrogenation in such a system gave 2-methyl-1,4-butanediol (97). 

CO, and methanol react in the first step in the presence of cobalt carbonyl catalyst and pyridine [110-86-1] to produce methyl pentenoates. A similar second step, but at lower pressure and higher temperature with rhodium catalyst, produces dimethyl adipate [627-93-0]. This is then hydrolyzed to give adipic acid and methanol (135), which is recovered for recycle. Many variations to this basic process exist. Examples are ARCO s palladium/copper-catalyzed oxycarbonylation process (136—138), and Monsanto s palladium and quinone [106-51-4] process, which uses oxygen to reoxidize the by-product... 

Ligand-Modified Cobalt Process. The ligand-modified cobalt process, commercialized in the early 1960s by Shell, may employ a trialkylphosphine-substituted cobalt carbonyl catalyst, HCo(CO)2P( -C4H2)3 [20161 -43-7] to give a significantly improved selectivity to straight-chain... 

The formation of isomeric aldehydes is caused by cobalt organic intermediates, which are formed by the reaction of the olefin with the cobalt carbonyl catalyst. These cobalt organic compounds isomerize rapidly into a mixture of isomer position cobalt organic compounds. The primary cobalt organic compound, carrying a terminal fixed metal atom, is thermodynamically more stable than the isomeric internal secondary cobalt organic compounds. Due to the less steric hindrance of the terminal isomers their further reaction in the catalytic cycle is favored. Therefore in the hydroformylation of an olefin the unbranched aldehyde is the main reaction product, independent of the position of the double bond in the olefinic educt ( contrathermodynamic olefin isomerization) [49]. 

The behaviour of the ruthenium catalysts is quite different from that previously reported for cobalt carbonyl catalysts, which give a mixture of aldehydes and their acetals by formylation of the alkyl group of the orthoformate (19). The activity of rhodium catalysts, with and without iodide promoters,is limited to the first step of the hydrogenation to diethoxymethane and to a simple carbonylation or formylation of the ethyl groups to propionates and propionaldehyde derivatives (20). 

T,he hydroformylation reaction or oxo synthesis has been used on an industrial scale for 30 years, and during this time it has developed into one of the most important homogeneously-catalyzed technical processes (I). A variety of technical processes have been developed to prepare the real catalyst cobalt tetracarbonyl hydride from its inactive precursors, e.g., a cobalt salt or metallic cobalt, to separate the dissolved cobalt carbonyl catalyst from the reaction products (decobaltation) and to recycle it to the oxo reactor. The efficiency of each step is of great economical importance to the total process. Therefore many patents and papers have been published concerning the problem of making the catalyst cycle as simple as possible. Another important problem in the oxo synthesis is the formation of undesired branched isomers. Many efforts have been made to keep the yield of these by-products at a minimum. 

Step 1 Formation of the Active Cobalt Carbonyl Catalyst. Many efforts have been made (2, 3, 4) to prepare carbonyl catalyst by treating metallic cobalt or cobalt compounds at high temperatures (150°-200°C) and pressures with carbon monoxide and hydrogen (Reactions 1 and 2). 

A novel method was reported for the carbonylation of aryl halides by cobalt salt catalysts, such as Co(OAc)2, CoCl2, C0SO4, Co(OH)2, Co(OH)3, CoO and Co203 in an aqueous alkaline solution and under irradiation214. 

In the case of the unpromoted cobalt carbonylation catalyst, a relatively clear picture as to the nature of the transformations is available. The original proposal of Wender et al. that the first step in the mechanism is the protonation of methanol by the strongly acidic HCo(CO)4 (48) has stood the test of time and is now generally accepted for this mechanism (Scheme 5). Subsequent migratory insertion yields the corresponding acyl derivative, which, when followed by hydrolysis by solvent water or alcohol, leads to the... 

Oxo process The commercial hydroformylation of an alkene by treatment with CO and H2 over, usually, a cobalt carbonyl catalyst. 

Ring expansion-carbonylation.1 The ring expansion of aziridines to /3-lactams by a Rh(l) catalyst (15,82-83) has been extended to expansion of pyrrolidines to piperidones by cobalt carbonyl-catalyzed carbonylation (equation 1). 

Work by Wayland et aL has shown that the low-valent rhodium compounds such as [(TXP)Rh]2 (TXP = tetra-(3,5-dimethylphenyl)porphyrinato), can reduce CO to form ethane dionyl compounds of the type [(TXP)Rh-C(O)-C(0)-Rh(TXP)] [97-100]. The late transition metals prefer to bind to carbon, and therefore this is formed in preference to an ethyne diolate product. Although carbocychc products are not observed, the reactions demonstrate that reductive homologation of CO is feasible with transition metals imder mild conditions. Related are the reactions of hydrosilanes with CO using rhodium and cobalt carbonyl catalysts, which give straight-chain products containing up to two or three coupled CO imits (however, at elevated pressmes and tem-peratmes) [101,102]. 

HCo(CO), an exceptionaUy acidic transition metal "hydride", is formed by tfie reaction of Hj and COjfCOlg. HCo(CO)g is a trigonal bipyramidal d , 18-electron complex in which the hydride occupies an apical position. High temperature is required for industrially useful rates of hydroformylation catalyzed by HCo(CO)g. Moreover, high pressure of CO is required to prevent formation of higher cobalt clusters and of metallic cobalt. The rate of hydroformylation catalyzed by cobalt carbonyl depends on [H ] and Thus, increasing the pressure of a 1 1 mixture of CO Hj has little effect on tire rate but prevents catalyst decomposition. 

The formation of isomeric 2-alkyl-branched aldehydes is caused by the cobalt organic intermediates, which are formed by the reaction of the olefin with the cobalt carbonyl catalyst. These intermediates isomerize rapidly into a mixture of isomer positions on the cobalt catalyst-olefin compound. 

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